docs: def get_closest(self, lon, lat, depth=0):
          """
          Get the closest object to the given longitude and latitude
          and its distance.
  
          :param lon: longitude in degrees
          :param lat: latitude in degrees
          :param depth: depth in km (default 0)
          :returns: (object, distance)
          """
          xyz = spherical_to_cartesian(lon, lat, depth)
          min_dist, idx = self.kdtree.query(xyz)
          return self.objects[idx], min_dist